1. Field of the Invention
The invention relates to a nonaqueous electrolyte and secondary cells employing the same, and more particularly to a nonaqueous electrolyte having maleimide additives and secondary cells employing the same.
2. Description of the Related Art
Along with the rapid development and availability of portable electronic products, demand for lithium ion secondary batteries, due to their properties, including having a light weight, high voltage, and high energy density, etc., have increased. Furthermore, the use of polymer electrolytes in lithium ion secondary batteries has become more and more important and is attracting wide attention in research for size reduction and increasing design flexibility for electronic products.
In addition, from the viewpoint of environmental protection and energy conservation, products are apt to be made by green materials (lead/halogen/phosphorus-free), in order to avoid causing environmental pollution and runaway greenhouse effect. Therefore, a battery, having high performance, high energy density, and environmental protection properties, is desired.
Chargeable secondary batteries have high electromotive force and high energy density. Lithium ion secondary batteries, commercialized by Sony Energy Tec., are in heavy demand for use as the main power supply for mobile communication devices and portable electronic devices. Lithium ion secondary batteries provide a high level of battery voltage, and have exceptional electrode characteristics, even though it is unsafe when charging or discharging at high rates, and is more costly than the other materials.
For the development of lithium ion secondary batteries, it is an important technical issue to enhance the reliability, capacitance, charge-discharge efficiency, and lifespan of the battery. Besides efficient matching of the electrode material and the isolation membrane, the most effective way to enhance lithium ion secondary batteries is to improve its electrolyte properties. In general, the electrolyte comprises lithium salt and a nonaqueous solvent. Since an ideal electrolyte should have the characteristics of high dielectric constant and low viscosity, cyclic carbonates (high dielectric constant) and linear carbonates (low viscosity) are thereby employed.
At present, an electrolyte simultaneously comprising thylene Carbonate propylene carbonate is commonly used in the industry and has a working voltage of 3.6V and an energy density of 250˜300 Wh/L (or 90˜110 Wh/kg). It has been discovered that by adding ethylene Carbonate in a propylene carbonate electrolyte system, the co-intercalation phenomenon of propylene carbonate with lithium occurs. Further, in the ethylene carbonate and propylene carbonate electrolyte system, improvements are necessary for thermal stability, flame retardancy and safety.
In order to solve the aforementioned problems, diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), vinylene carbonate, sulfites, sulfates, or phosphate was added to the ethylene carbonate and propylene carbonate electrolyte system, thereby preventing the dissolution of electrolytes and improving performance.
Although by merely adding the previous mentioned additives, the properties of the batteries can be improved to a certain degree, the effect however, is not very distinctive. Often, a certain property is improved and others are not affected. Since the additives are often costly, and some of the additives require a large amount to be used when individually applied, costs are greatly increased.
As the market and customers increase their demand for high capacity and practical usage properties in different operating conditions, the industry is pushed to design better products. Therefore, it is distinctively important to find an additive with good overall properties, or a combination of additives which can improve various properties of batteries.